Crystallisation of spray-dried amorphous precursors in the SrO-Al2O3 system:: a DSC study

The crystallisation of amorphous precursors in the SrO-Al2O3 system has been studied by DSC and XRD. Amorphous precursors were obtained for compositions ranging from 12SrO.7Al(2)O(3) to Al2O3 by spray-drying aqueous solutions of strontium and aluminium nitrates followed by heating the powders to decompose the nitrates. Of the stable compounds SrA, SrA(2) and SrA(6), only SrA crystallises directly at a usual heating rate of 5 degreesC/min (in this notation Sr = SrO and A = Al2O3). All the alumina-richer compositions are characterised by the first and strong crystallisation of metastable solid solutions of SrA in its hexagonal form, stabilised by excess aluminium cations, or gamma-Al2O3 Stabilised by strontium cations, or even the two solid solutions, depending on the composition. The hexaaluminate SrA(6) is formed as single phase by an exothermic transformation of the gamma-Al2O3 solid solution at 1183 degreesC. The formation of SrA(2) is complicated by the easy and competitive formation of Sr(4)A(7), of close composition. The latter compound is stable only up to 1500 degreesC and is decomposed at higher temperatures. However SrA(2), which is characterised by a slow kinetics of formation from the amorphous state as well as from the high temperature liquid state, may be obtained pure by slow heating or annealing at 900-1000 degreesC. (C) 2003 Elsevier Science Ltd. All rights reserved.